AMPK has a key role in regulating the energy metabolism in eukaryotic cells and is homologous to the SNF1 kinase in yeast (Hardie D. G., et al., 1998, Annu. Rev. Biochem. 67:821; Kemp B. E., et al., 1999, Trends. Biochem. Sci. 24(1):22–5). AMPK is composed of three subunits: the catalytic α-subunit and the two regulatory subunits β and γ. AMPK is activated by an increase in the ratio of AMP to ATP (AMP:ATP). Activated AMPK turns on ATP-producing pathways and inhibits ATP-consuming pathways. AMPK also can inactivate glycogen synthase, the key regulatory enzyme of glycogen synthesis, by phosphorylation (Hardie et al., 1998, supra). Several isoforms of the three different AMPK subunits are present in mammals. In humans, Prkaa1 and Prkaa2 encode the α1 and α2 subunits, Prkab1 and Prkab2 encode the β1 and β2 subunits, and Prkag1, Prkag2 and Prkag3 encode the γ1, γ2 and γ3 subunits, respectively.
Milan D., et al. (2001, Science, 288:1248–5) identified the nonconservative substitution of a glutamine for an Arginine (R225Q) in the Hampshire pig Prkag3 gene responsible for the dominant RN-phenotype (high glycogen content in skeletal muscle). Loss-of-function mutations in the homologous gene in yeast (SNF4) cause defects in glucose metabolism, including glycogen storage. Milan et al. further found that the expression of the Prkag3 gene is muscle-specific and that the AMPK activity in muscle extracts was about 3 times higher in normal rn+ pigs than in RN− pigs, both in the presence and absence of AMP. The distinct phenotype of the RN-mutation indicates that Prkag3 plays a key role in the regulation of energy metabolism in skeletal muscle.
AMPK is recognized as a major regulator of lipid biosynthetic pathways due to its role in the phosphorylation and inactivation of key enzymes such as acetyl-CoA carboxylase (ACC) (Hardie D. G., and Carling D., 1997, Eur. J. Biochem. 246:259–273). More recent data strongly suggest that AMPK has a wider role in metabolic regulation (Winder W. W., and Hardie D. G., 1999, Am. J. Physiol., 277:E1–E10): this includes fatty acid oxidation, muscle glucose uptake (Hayashi T., et al., 1998, Diabetes, 47:1369–1373; Merrill G. F., et al., Am. J. Physiol. 273:E1107–E1112; Goodyear L. J., 2000, Exerc. Sport Sci. Rev., 28:113–116), expression of cAMP-stimulated gluconeogenic genes such as PEPCK and G6Pase (Lochhead P. A., et al., 2000, Diabetes, 49:896–903), and glucose-stimulated genes associated with hepatic lipogenesis, including fatty acid synthase (FAS), Spot-14 (S14), and L-type pyruvate kinase (Foretz M., et al., 1998, J. Biol. Chem., 273:14767–14771). Chronic activation of AMPK may also induce the expression of muscle hexokinase and glucose transporters (Glut4), mimicking the effects of extensive exercise training (Holmes B. F., et al., 1999, J. Appl. Physiol. 87:1990–1995). Thus, it has been predicted that AMPK activation would be a good approach to treat type 2 diabetes (Winder et al., supra).
Zhou G, et al. (2001, J. Clin. Invest., 108:1167–1174) provided evidence that the elusive target of metformin's (a widely used drug for treatment of type 2 diabetes) actions is activated AMPK. In studies performed in isolated hepatocytes and rat skeletal muscles, metformin leads to AMPK activation, accompanied by an inhibition of lipogenesis (due to inactivation of acetyl-CoA carboxylase and suppression of lipogenic enzyme expression), suppression of the expression of SREBP-1 (a central lipogenic transcription factor), and a modest stimulation of skeletal muscle glucose uptake. Similar hepatic effects are seen in metformin-treated rats. Based on the use of a newly discovered AMPK inhibitor, their data suggest that the ability of metformin to suppress glucose production in hepatocytes requires AMPK activation.